Earth's Layers: Crust, Mantle, and Core

Layers of the Earth

• The Earth consists of the crust, mantle, outer core, and inner core, each with distinct physical and chemical properties.
• The crust, the Earth's outermost solid layer, ranges in thickness from about 5 km (oceanic) to 70 km (continental).
• Oceanic crust is mainly basalt, a dense volcanic rock.
• Continental crust consists primarily of granite, a less dense igneous rock.
• The mantle extends to a depth of about 2,900 km from the base of the crust.
• Approximately 84% of the Earth's volume is made up of the mantle, making it the thickest layer.
• Silicate rocks rich in iron and magnesium are the primary components of the mantle.
• The mantle is divided into the upper and lower sections based on mineral composition and physical properties.
• The lithosphere, which consists of the uppermost part of the mantle and the crust, is rigid and fractured into tectonic plates.
• The asthenosphere, a partially molten layer beneath the lithosphere, allows the lithospheric plates to move.
• The outer core, which is approximately 2,200 km thick and composed mainly of iron and nickel, is a liquid layer.
• Earth's magnetic field results from the movement of liquid iron in the outer core, known as the geodynamo effect.
• The inner core is a solid sphere with a radius of approximately 1,200 km, primarily made of iron and nickel.
• The inner core remains solid at high temperatures because of extreme pressure.

Theory of Continental Drift

• Alfred Wegener proposed this theory in the early 20th century.
• The continents were once part of a single landmass called Pangaea.
• Pangaea started breaking apart about 200 million years ago.
• The continents gradually moved to their present locations.
• Evidence includes:
  • The fit of continents (e.g., South America and Africa).
  • Similar fossil plants and animals on different continents.
  • Matching rock types and geological structures across continents.
  • Evidence of past glaciation in unexpected locations.
• The theory was initially rejected due to a lack of a convincing mechanism for continental movement.

Theory of Plate Tectonics

• Based on continental drift and seafloor spreading, it was developed in the 1960s.
• The Earth's lithosphere is divided into several plates, both large and small.
• These plates move relative to each other, floating on the semi-molten asthenosphere.
• Plate boundaries are zones of intense geological activity, where earthquakes, volcanoes, and mountain building occur.
• Types of plate boundaries:
  • Divergent: Plates move apart, creating new crust (e.g., mid-ocean ridges).
  • Convergent: Plates collide, leading to subduction or mountain building (e.g., Himalayas, Andes).
  • Transform: Plates slide past each other horizontally (e.g., San Andreas Fault).
• Plate movement is driven by convection currents in the mantle and slab pull.
• Convection is the rising of hot material and the sinking of cooler material.
• Slab pull is the force exerted by a subducting plate as it sinks into the mantle.

Volcanoes

• Volcanoes are geological formations where magma erupts onto the Earth's surface.
• They are formed by different processes.
• Magma is generated by:
  • Decompression melting at divergent boundaries and hotspots.
  • Addition of volatiles (e.g., water) at subduction zones.
  • Heat transfer from rising magma to the surrounding crust.
• Types of volcanoes:
  • Shield volcanoes: Broad, gently sloping volcanoes formed by fluid basaltic lava flows (e.g., Hawaiian Islands).
  • Cinder cones: Small, steep-sided volcanoes formed by explosive eruptions of pyroclastic material (e.g., Paricutin, Mexico).
  • Composite volcanoes (stratovolcanoes): Large, cone-shaped volcanoes composed of alternating layers of lava and pyroclastic material (e.g., Mount Fuji, Mount St. Helens).
• Eruptions can be effusive (lava flows) or explosive (pyroclastic flows, ash clouds).
• Volcanic hazards include:
  • Lava flows: Destructive but generally slow-moving.
  • Pyroclastic flows: Hot, fast-moving currents of gas and volcanic debris.
  • Ash falls: Widespread deposits that can disrupt air travel, damage infrastructure, and affect human health.
  • Lahars: Mudflows composed of volcanic ash, rock, and water.
  • Volcanic gases: Release of toxic gases such as sulfur dioxide and carbon dioxide.

Rock Formation

• Rocks are naturally occurring aggregates of minerals.
• The three major types are igneous, sedimentary, and metamorphic.
• Igneous rocks are formed by the cooling and solidification of magma or lava.
  • Intrusive igneous rocks: Cool slowly beneath the Earth's surface, resulting in large crystals (e.g., granite).
  • Extrusive igneous rocks: cool rapidly on the Earth's surface, resulting in small crystals or a glassy texture (e.g., basalt).
• Sedimentary rocks are formed by the accumulation and cementation of sediments.
  • Clastic sedimentary rocks: Formed from fragments of other rocks (e.g., sandstone, shale).
  • Chemical sedimentary rocks: Formed by precipitation of minerals from solution (e.g., limestone, rock salt).
  • Organic sedimentary rocks: Formed from the remains of plants and animals (e.g., coal).
• Metamorphic rocks are formed when existing rocks are changed by heat, pressure, or chemical reactions.
  • Foliated metamorphic rocks: Have a layered or banded appearance (e.g., gneiss, schist).
  • Non-foliated metamorphic rocks: Do not have a layered appearance (e.g., marble, quartzite).
• The rock cycle describes how rocks transform from one type to another.

Tsunami

• Tsunamis are large ocean waves caused by sudden displacement of water.
• Common causes:
  • Underwater earthquakes.
  • Volcanic eruptions.
  • Landslides.
  • Meteorite impacts (rare).
• Tsunami waves have long wavelengths (hundreds of kilometers) and travel at high speeds (up to 800 km/h) in the open ocean.
• As a tsunami approaches the coast:
  • speed decreases.
  • height increases dramatically.
• Coastal effects:
  • Flooding.
  • Erosion.
  • Destruction.

Earthquakes

• Earthquakes are vibrations in the Earth caused by the sudden release of energy.
• Most occur along plate boundaries.
• Faults are fractures in the Earth's crust where movement has occurred.
• Elastic Rebound Theory:
  • Stress builds up along a fault as plates move.
  • The fault ruptures when the stress exceeds the strength of the rocks.
  • Energy is released as seismic waves.
• Focus (hypocenter): The point within the Earth where the earthquake originates.
• Epicenter: The point on the Earth's surface directly above the focus.
• Seismic waves:
  • P-waves (primary waves): Compressional waves that travel through solids, liquids, and gases.
  • S-waves (secondary waves): Shear waves that travel through solids only.
  • Surface waves: Travel along the Earth's surface and cause the most damage.
• Earthquake magnitude:
  • Richter scale: Measures the amplitude of seismic waves on a logarithmic scale.
  • Moment magnitude scale: Measures the total energy released by an earthquake.
• Earthquake hazards:
  • Ground shaking.
  • Liquefaction.
  • Landslides.
  • Tsunamis.

Subduction

• Subduction is a process where one tectonic plate slides beneath another at a convergent boundary.
• Typically, an oceanic plate collides with a continental plate or another oceanic plate.
• The denser oceanic plate subducts beneath the less dense plate.
• Subduction zones are characterized by:
  • Deep-sea trenches.
  • Volcanic arcs.
  • Earthquakes.
• Examples:
  • The Andes Mountains: Formed by the subduction of the Nazca Plate beneath the South American Plate.
  • The Japanese Islands: Formed by the subduction of the Pacific Plate beneath the Eurasian Plate.
• The subducting plate releases water and other volatiles into the mantle, which lowers the melting point and generates magma.
• The magma rises, forming volcanic arcs.
• Subduction plays a role in the formation of new continental crust and the recycling of materials from the Earth's surface back into the mantle.